The present invention relates to mounting means for a substrate in a microcircuit device and to a microcircuit assembly in which a carrier is utilized for mounting a substrate with respect to a housing.
Microcircuit devices include at least one substrate in a housing. As used in the present description, the term "substrate" means a block of material, such as beryllium oxide, having a hybrid circuit or other circuit means formed on an upper surface thereof. Another term typically used to characterize substrates in the art is "chips." Substrates must be reliably mounted in housings for purposes of electrical characteristics, mechanical integrity, and proper heat transfer from the substrate. Commonly, substrates are mounted to a housing or to a bonding pad in a housing, thus becoming permanently affixed to the housing. Certain disadvantages arise from this conventional procedure. An example is a microwave transmitter including a plurality of substrates. In one form of typical transmitter, five substrates may be mounted in a housing. Each substrate is tested individually, and then bonded in a housing. Once bonded, interconnections are provided between substrates and from one or more substrates to housing terminals. Once completely assembled, the microwave transmitter must be tested. If individually operative substrates do not interact properly, or if a substrate becomes defective, it is necessary to remove one or more substrates. Removal of one or more substrates can result in permanent damage to substrates such that they are no longer usable. Removal operations may also endanger the operability of other substrates in the housing. This becomes particularly significant in a typical microwave transmitter applications, for example, in which the cost of producing a machined housing may be $200, and the cost of substrates placed thereon may be $8,000. It is therefore desirable to provide means for interchangeability of substrates in a housing.
Additionally, it is highly important that a reliable interface be provided between a lower surface of a substrate and the mounting means to which it is bonded. Substrates are generally made of fired ceramic material having a flat lower surface. While the lower surface may remain flat after firing, there is a tendency to warp. In a typical substrate one inch long, there may be as much as 0.0015 inches of twist in the substrate with respect to a flat plane. Even such a low degree of twist may result in an interface between the lower surface of the substrate and a mounting surface wherein portions of the substrates do not touch the mounting surface. Areas of discontinuity of mating, though small, may adversely affect electrical operations of the substrate, particularly in the case of substrates comprising microwave circuits. While voids may be filled by bonding material, the use of excess bonding material may cause disontinuities due to non-uniform distribution. In some forms, a substrate has a ground plane formed on a lower surface thereof. The ground plane may consist of a layer of gold alloy. It is desirable to prevent damage to this layer during bonding. If gold is removed from the ground plane during bonding, electrical properties of the substrate may be adversely affected.